Change a `cp` command.
Change a `cp` command.

The existing command used is `cp -r -T -- src_dir dst_dir`. But
-T doesn't work on Mac.

This commit changes it to `cp -r src_dir/. dst_dir`.

- Appending the `.` has the same effect as -T: copy the contents of
  `src_dir` without copy `src_dir` itself.

- Also, -R is POSIX but -r is not.

nrc/rustc-perf,nrc/rustc-perf,nrc/rustc-perf